are we talking about mexico or the dominican republic? if we are talking mexico than do a search, i have a post a while back about my experience there which should pretty much sum up everything you will need to do. i went to tiajuana, can't speak for the rest of mexico.
you CAN buy hydrocodone, oxycontin, roofies, and valium. i found all of those in pharmacies there. i'm pretty sure its really easy to get k too, but i didn't try. a large amount have valium/diazapam, those who don't will offer soma instead. i did find one place with rophynol(sp?), and a couple with hydrocodone and oxycontin. two bad they were asking $15 for a 10mg of oxycontin! fucking rip off. both places that had hydrocodone charged $2 a peice all the way up. best i found on valium was $60 for 90x10mg if i remember correctly,. and that was a good deal, most places won't negotiate at all either, not like the rest of that flea market of a town. there are about a billion pharmacies there so just walk up to the people out front, you'll know who i'm talking about when you get there and see them, and flat out ask if they have whatever your looking for. if not, go to the next one which will be about three stores down.